Method of producing magnesium hydroxide having variable particle sizes



United States Patent No Drawing. Filed Sept. 28, 1962, Ser. No. 227,034

2 Claims. (Cl. 23-201) This invention relates to the production ofmagnesium hydroxide having variously sized particles when produced bythe reaction of lime or dolime with sea water-derived solutionscontaining a soluble magnesium halide, whereby MgO of controlledparticle size can be produced.

Magnesia (MgO) may be produced by the calcination of an ore such asmagnesite. However, sea water-derived magnesia is a higher qualitymaterial. One of the problems which has arisen in the production ofmagnesium hydroxide from sea Water is the difficulty in obtainingcontrolled particle sizes which is essential for almost all its uses.This, in turn, is due to the fact that, in general, the magnesiumhydroxide which is formed by mixing magnesium chloride-containingsolutions with lime or dolime is a fine floc having a random particlesized distribution.

It is desirable to control the particle size of the magnesium hydroxide,since the grade of MgO which is obtained by calcining the magnesiumhydroxide is correlated to the size of the particle entering thecalciner. Different grades of magnesia (MgO), each having differentphysical properties, are obtained upon calcination of magnesiumhydroxides having different particle sizes because the large particlesizes have a physical structure which differs from magnesium hydroxidehaving small particle sizes. Accordingly, a difference in the size ofthe magnesium hydroxide particle which is precipitated reflects a changein the properties of the particle which is not merely a function ofsize. Control of the particle size of the Mg(OH) permits the controlledproduction of various grade of magnesia for a multitude of commercialuses.

One method for sizing these particles to give fractions having varyingcharacteristics is taught in US. Patent 2,703,273, issued to James AllenRobertson et al. on March 1, 1955. However, this process does notattempt to control the size of the precipitated magnesium hydroxideproduct. It merely separates the existing fine particles into variouslysized fractions.

Another problem which has arisen in the production of magnesiumhydroxide derived from sea water or bitterns is the relatively long timerequired for the magnesium hydroxide sludge to separate from thesolution. This is due to the relatively fine floc and low settleddensity which results when ordinary magnesium chloride solutions and thelime or dolime are merely reacted together. Reduction of the settlingtime is commercially advantageous in order to reduce the size and costof the large settling tanks which are presently required to permitrecovery of the fine M'g(OH) precipitate.

It is an object of the present invention to control the size ofmagnesium hydroxide particles produced by reacting magnesium solutionsand either dolime or quicklime so that a coarse, large particle sizedfraction having a rapid settling rate can be obtained in addition to afine particle sized fraction of high purity.

This and other objects will become apparent from the followingdisclosure.

It has now been found unexpectedly that the size of magnesium hydroxideparticles can be controlled by mixing a magnesium halide-containingsolution with either quicklime or dolime in amounts sufficient to reactwith .could be produced otherwise.

from about 10% to of the magnesium values contained in the magnesiumhalide solution in the presence of a soluble boron salt in amountssufficient to supply from 20 to 500 mg. of boron per liter of suchsolution, precipitating a coarse Mg(OH) initial fraction, separatingthis coarse magnesium hydroxide sludge from the resultant supernatantmagnesium halide solution, reacting the resultant supernatant solutionwith additional amounts of quicklime or dolime up to about to 99% of thestoichiometric amount required to react with the remaining magnesiumvalues, precipitating an additional magnesium hydroxide product fractionin the presence of less boron than was present in the initialprecipitation, this fraction having a finer particle size than theinitial coarse fraction of magnesium hydroxide, and separating the finerMg(OH) fraction from the mother liquor substantially reduced inmagnesium values.

The reason for the larger particle size of the initially precipitatedmagnesium hydroxide product, as compared with the latter M,g(OH)fraction, is the presence of the given amount of soluble boron in thesolution during the initial precipitation. The soluble boron and otherimpurities are removed from the solution along with the initial Mg(OH)precipitate, permitting the fine Mg(OH) fraction to be precipitated inthe supernatant liquor in the absence of substantial amounts of solubleboron and other impurities normally found in the solutions. Byincreasing the concentration of the boron solution, the particle size ofthe initially precipitated magnesium hydroxide fraction becomes coarser.This increased particle size reaches a leveling off effect when theconcentration of the boron reaches about 500 milligrams per liter ofsolution. The initial coarse fraction of Mg(OH) which is recovered hasbeen found to contain sizable amounts of boron and iron derived from thebitterns and impurities derived from the dolime. This grade of Mg(OH)upon calcination to MgO, is suitable as an example, for use in magnesiumoxychloride cements. The fine fraction of Mg(OH) contains substantiallyreduced amounts of boron, iron or other impurities derived from thebitterns, in contrast to the coarse fraction. In addition, the majorimpurities derived from the dolime such as Fe O SiO A1 0 and unreactedCaO can be separated readily from the fine fraction of Mg(OH) by asimple sizing operation, e.g. cyclone separator, to yield a purer gradeof magnesia than Separation is facilitated by the coarseness of thedolime-derived impurities compared with the finer Mg(OH) precipitate.This finer grade of sized Mg(OH) upon being calcined to MgO, is suitablefor use as a higher purity chemicma grade MgO.

A two-stage operation which is the preferred embodiment, is carried outas follows: Dolime or quicklime is added to a magnesium salt solutioncontaining a water soluble boron salt in amounts sufficient to supplyfrom about 20 to about 500 milligrams of boron per liter of solution.The amount of dolime or quicklime which is added is substantially lessthan the stoichiometric equivalent of calcium oxide required to act withall the magnesium in solution. The quantity of dolime or quicklime whichis reacted in the first stage of the process will be determined by thedesired degree of coarseness of the primary magnesium hydroxide sludgeand by the boron content of the magnesium salt solution. It can be addedin amounts sufficient to react with from between 10% to 90% of themagnesium values contained in the magnesium solution although it ispreferred that enough be added so that only about 50% of the magnesiumion con tained in the magnesium solution be precipitated in the firststage. Upon separation of the magnesium hydroxide sludge from thesupernatant liquor containing soluble magnesium values, the supernatantliquor is reacted with additional quantities of quicklime ordolime inamounts to precipitate all but about 1 to 5% of the magnesium values inthe magnesium halide solution.

The resulting magnesium hydroxide, precipitated in the second stage, hasa much finer particle size than the initially precipitated magnesiumhydroxide product. All of the remaining magnesium ions in the magnesiumhalide solution are not precipitated in the second stage in order toallow the calcium oxide in the dolime or quicklime to react sothatmagnesium hydroxide is precipitated with as little calciumcontamination as possible.

The magnesium-containing solution employed can be sea water, bitterns orany other solutioncontaining recoverable magnesium values. Bitterns, aregenerally mother liquor left after evaporation of sea water or brinesfor the recovery of common salt. These solutions often contain residualsodium chloride and sulfate salts. It is usually advantageous to treatthe bitterns with an excess of a soluble calcium salt to remove thesulfate values as the relatively insoluble calcium sulfate. Theresultant solution is termed MgCl bitterns and is more desirable thanthe raw bitterns since it contains less impurities to contaminate thefinal product. These MgCl bitterns generally contain about 40 to 200grams of MgCl per liter of solution.

The boron compoundswhich have been found operable are water solubleboron salts. The boron salts all have the, same effect regardless of theparticular salt employed so long as they can be dissolved in the aqueousmagnesium-containing solution. Pure and commercial grades of compoundssuch as boric acid, sodium perborate, borax, boric oxide and boricanhydride have all been found suitable. The boron compound must bepresent in the solution, when magnesium hydroxide is precipitated, inamounts to supply from about 20 to 500 milligrams of boron per liter ofsolution. Amounts lower than about 20 milligrams of boron per liter maybe employed, but the particle size of the precipitated Mg(OH) isincreased only slightly. Amounts over 500 milligrams of boron per litercan be employed. However, these higher amounts are not effective inincreasing the particle size of the Mg(OH) beyond that obtained bysolutions containing boron in amounts up to 500 milligrams per liter.

Although either quicklime or dolime .can be employed for reaction withthe magnesium ..halide-containing solution, dolime is generallypreferred. Dolime is the term applied to dolomite having a calcium tomagnesium mole ratio of about 1, or to dolomitic limestone which is anaturally occurring mixture of limestone and either dolomite andmagnesite or both, having a calcium to magnesium mole ratio greater thanunity, which has been calcined so as to be essentially free of residualCO and in which the CaO is in a reactable state. Quicklime is the termapplied to the reactable product obtained by calcining materials whichare composed primarily of calcium carbonate, such as limestone, theshells of mollusks, etc. Dolime is preferred because in addition tocalcium oxide, it also contains magnesium values which are usuallyrecovered along with the magnesium values present in the sea water,bittern, or other magnesium-containing solution.

This two-stage process for producing magnesium hydroxide is advantageousbecause it allows the production of essentially two size ranges ofmagnesium hydroxide particles, the second magnesium hydroxide sludgealways having finer particles than the initially precipitated magnesiumhydroxide sludge. Further, the size of the particles of magnesiumhydroxide which are initially precipitated can be controlled byregulating the amount of boron which is added to the solution in whichthe magnesium hydroxide precipitates. Large amounts of boron, on theorder of 100 to 500 mg./l., have been found to produce extremely largeparticles. In addition, if the amount of dolime or quicklime which isadded is progressively reduced in the initial reaction stage, thecoarseness of the resulting magnesium hydroxide sludges willprogressively increase to a limiting value when the amount of dolime orquicklime corresponds to about 10 to 15% of the magnesium ion containedin the magnesium halide solution.

The greatest difierence in degree of coarseness between the primary andsecondary sludges is obtained when approximately one quarter to one halfthe magnesium ions contained in the magnesium halide solution isprecipitated in the primary reaction, and essentially all of theremaining magnesium ions are precipitated in the secondary step.However, significant contrasts in sizes between the primary andsecondary sludges are obtained when 20% to of the magnesium ions areprecipitated in the primary reaction and essentially all of theremaining magnesium ions are precipitated in the secondary reaction.

The difiference in particle size between the primary and secondaryreaction mixtures can only be brought about by the inclusion ofsufficient soluble, boron compound to afiect the size of the magnesiumhydroxide particle. If little or no soluble boron is present, thetwo-stage addition of dolime or quicklime does not result in theproduction of primary and secondary magnesium hydroxide products havingsignificantly difierent particle size distributions. Instead, only auniform size distribution of particles is obtained which is essentiallythe, same in both the primary and secondary sludges.

The present process can be carried out continuously using separatereactors and settling tanks. On the other hand, if storage for thepartially spent liquor is provided, the two magnesium hydroxide sludgescan be prepared in the same equipment by sequential reactions andseparations.

One method for measuring the relative sizing of the magnesium hydroxidesludge is to determine its fineness modulus. The fineness modulus isdefined as the sum of each of the cumulative weight percent retained byeach sieve in a series of filter sieves divided by the number of sievesin the series. Thus, in comparing two sludges, the one with the higherfineness modulus will be the coarser. As used in the specification andexamples, the fineness modulus is based on the Tyler Standard of Sieves.having the following sieve numbers: 28, 32, 48, 65, 100, 150, 200, and325.

While the process has been described with reference to the preferredembodiment, namely, a two-step precipitation, it is considered that thepresent process is readily adaptable to the production of an initialcoarse fraction and two or more finer fractions of Mg(OI-1) This can becarried out by precipitating an initial Mg(OH) coarse fraction asdescribed above, and separating the precipitate from the supernatantliquor. Sufiicient lime or dolime is then added to the supernatantliquor to precipitate a first fine fraction in aliquot which containsless boron than was present when the coarse fraction was precipitated.However, this lime or dolime is added in amounts to react with only aportion of the remaining magnesium values in the liquor. The finefraction is separated from its supernatant liquor and additional lime ordolime is added to the supernatant liquor to recover a second fine.fraction of Mg(OH) in the presence of less boron than was present wheneither the coarse fraction or the first fine: fraction was precipitated.By this means, and by subsequent size separations, fine fractions ofincreasing chemical purity can be obtained, which upon calcination toMgO, yield magnesias of varying chemical purity and sizes.

This multi-step operation can readily be carried out where largequantities of boron are present in the original magnesium halidesolution. In such cases, the residual supernatant. liquor separated fromthe initial coarse pre cipitate would contain sufi'icient boron topermit varying grades of magnesium hydroxide in subsequentlyprecipitated Mg(OH) fractions. Magnesia products derived from thesedifferent grades of magnesium hydroxide precipitates would have varyingproperties, enabling them to be more readily accepted for special usesin which general purpose magnesias have not been found acceptable.

The following examples are given by way of illustration only and are notdeemed to be limiting to the invention.

EXAMPLE I Milled dolime was reacted with a MgCl bittern solutioncontaining naturally derived soluble boron. The MgCl bittern wassulfate-free and derived from the solar evaporation of sea water. Itcontained 73 g. MgCl /l., 23 g. CaCl /L, 72 g. NaCl/l., 19 g. KCl/L, and0.065 g. B/l. The dolime had a fineness modulus of 25 and was added intwo stages. Enough dolime was added so that 47% of the Mg++ contained inthe bittern was precipitated as Mg(OH) in the primary stage. Afterseparating the partially spent liquor from the sludge, sufiicient dolimewas added in the second stage of the reaction to precipitate 99% of theMg remaining in the partially spent liquor. The coarse,,primary sludgeand the fine, secondary sludge were maintained in separate vessels forwashing and further treatment. In each stage of the process, thereaction between the dolime and the solution was caused to occur by acontinuous, gentle agitation of the suspension of solid particles for aperiod of six hours, followed by a settling period. After settling for16 hours, the spent liquor was separated from the settled sludge and thesludge was repulped to a uniform dispersion. The solids content ofthe-sludge (settled density) was determined by dissolving a measuredvolume in an excess of a standard .5 normal HCl solution in the presenceof approximately 2 g. NH Cl, then titrating the excess acid to theendpoint of a suitable indicator such as Methyl Orange modified byXylene Cyanole with a standard 1 normal N aOH solution.

The particle size distribution of the sludge was determined by pouring ameasured volume of known solids content into a nest of sieves, thenwashing the material through the screens with a gentle stream of wateruntil the effluent was free of turbidity. The amount of Mg(OH) retainedby each sieve was determined by acidimetrically, and expressed as aweight percent of that contained in the sample used for the test. Theparticle size distribution, fineness modulus, and settled density aregiven in Table 1.

EXAMPLE II A series of runs was made in which the total amounts ofdolime which were added were distributed between a primary and asecondary reaction in weight proportions of 75/25, 50/50, and 25/75.Additionally, a run was made in which all of the dolime was reacted in aprimary step with no subsequent secondary step (i.e., a dolimedistribution of 100/0). These runs were carried out as follows:sulfate-free bittern derived from the solar evaporation of sea water,and containing 73 g. MgCl /l., 23 g. CaCl /L, 72 g. NaCl/l., 19 g.KCl/l., and 0.065 g. B/l. was reacted in two stages with dolime milledto pass a 20 mesh sieve and having a fineness modulus of 25. Theproportions of dolime and bittern were selected so that a total of 95%of the Mg++ contained in the bittern would be precipitated as Mg(OH)cumulatively in the primary and secondary reactions. It was calculatedthat 212 g. of dolime would be required for reaction with 3 liters ofbittern in order to precipitate 253 g. of Mg(OH) including that obtainedfrom the hydration of the MgO values of the dolime. The 212 g. of dolimewhich was to be added was then divided into the two portions astabulated in Table 2. One portion was dispersed in the bittern and thesuspension of Mg(OH) which resulted was agitated for a period of sixhours. The slurry was then permitted to settle for approximately 16hours with the formation of predominantly coarse primary Mg(OH) sludgeand a clear liquid containing reduced concentrations of Mg++ and boron.The partially spent liquor was separated from the sludge. The remainingportion of the dolime was added to precipitate a second Mg(Ol-I) sludge.This was treated in the same manner as the primary sludge product. Theparticle size distributions and the settled densities of the sludges foreach of the runs were determined in the manner described in Example I.The results are tabulated in Table 2.

Table 2 Primary Secondary Reaction Stage-D olime Distribution ReactingSolution, mg.

B l 64. 7 64. 7 64. 7 64. 7 7. 8 8.4 19.0 Decanted Solution, mg.

4.0 7.8 8.4 19.0 2.2 0.6 2.2 Sieve Analyses of Sludge,

percent retained, cumulative:

.4 1.9 3.7 11.7 1.3 0.6 1.1 .3 3.3 5.6 14.8 2.3 1.2 2.1 3 8.7 12.7 22. 67. 4 5. 4 5.9 3 14. 9 20. 0 29. 9 12. 9 7. 8 7. 1 7 25. 3 31. 8 40. 014. 2 9. 2 9. 7 1 37.1 43. 5 50. 8 16. 3 12.1 12.9 .9 46. 9 53.9 59. 117. 6 14.6 17. 5 .4 58. 2 65. 2 66. 8 21. 2 17. 5 25.2 Fineness Modulus16. 6 24. 5 29. 6 37.0 11.6 8. 6 10. 2 Settled Density,

Mg(OI:I) g./1 327 364 353 327 132 182 214 EXAMPLE III The followingexample was run to demonstrate the correlation between the boron contentof the magnesium salt solution and the sizing and settled density of theinitially precipitated Mg(OH) sludge. A series of runs was made usingaqueous solutions containing 69 g. MgCl /l., 21 g. CaCI /L, 52 g.NaCl/l. and 18 g. KCl/l. Selected amounts of either borax (Na B O'r10HO), boric acid (H BO or sodium perborate (NaBO -H O -3H O) as reportedin Table 3, were dissolved in 3 liters of each of the solutions. Theresulting boron-containing solutions were then reacted with 202 g. of arnilled dolime containing 58% CaO and having a fineness modulus of 27.This quantity of dolime was suificient to react with 96% of the Mg++contained in the solutions. The Mg(OH) precipitate was permitted tosettle for about 16 hours in the manner described in Example II, and thenearly spent liquor was decanted from the settled sludge and discardedwithout further treatment for recovery of a second Mg(OH) product. Theparticle size distributions of the precipitate were determined in thesame manner as in Example II. The results ob tained from these runs anda control containing only a trace of boron, are listed in Table 3.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat, within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and

specifically described and exemplified herein.

Table 3 What is claimed is:

halide solution, precipitating a magnesium hydroxide 1. In the processof pr fraction having a finer particlie size' 'tha n the initial fracbythe reaction of a mag oducing magnesium hydroxide nesiumhalide-containing solution f magnesium hydroxide and separating saidfiner magnesium" hydroxide fractionfrorn said'mother liquor which isSubstantially reduced in magnesium values.

tion with a compound selected from the class consisting of quicklime anddolirne, the improvement which com- 2. Process ofclaim 1' in whichthe'co mpound selected from the group consisting of quicklime anddolirne is initially added in amounts suflicient "to react with 'fromprises adding said compound selected from the group conabout to about5.0% of .the magnesiuin values contained in the magnesium solution,

amine-r per liter, precipitating a relatively coarse Mg(OI-I) fraction,separating said coarse fraction of Mg(OH) sludge from the resultantsupernatant magnesium halide solu- I u w e. fi in .o I Se ame w enrD D OWLCR 5 3 4 45 9990/ 11111 9 1 3 364 3 8 6 3 7 ,95 00573 7 20 2 2 2 2 2 Otion reduced in both magnesium and boron values, reacting saidsupernatant magnesium halide solution with an additional quantity of acompound selected from the group consisting of dolirne and quicklime inamounts suflicient to precipitate no more than to 99% of the 35 MAURICEA. BRINDISI, Primary Examiner. remaining magnesium in said supernatentmagnesium

1. IN THE PROCESS OF PRODUCING MAGNESIUM HYDROXIDE BY THE REACTION OF AMANGESIUM HALIDE-CONTAINING SOLUTION WITH A COMPOUND SELECTED FROM THECLASS CONSISTING AT QUICKLIME AND DOLIME, THE IMPROVEMENT WHICHCOMPRISES ADDING SAID COMPOUND SELECTED FROM THE GROUP CONSISTING OFQUICKLIME AND DOLIME IN AN AMOUNT SUFFICIENT TO REACT WITH FROM ABOUT10% TO ABOUT 90% OF THE MAGNESIUM VALUES CONTAINED IN THE MAGNESIUMHALIDE SOLUTION IN THE PRESENCE OF A SOLUBLE BORON SALT IN AMOUNTSSUFFICIENT TO SUPPLY FROM 20 TO 500 MILLIGRAMS OF BORON PERLITER,PRECIPITATING A RELATIVELY COARSE MG(OH)2 FRACTION, SEPARATINGSAID COARSE FRACTION OF MG(OH)2 SLUDGE FROM THE RESULTANT SUPERNATANTMAGNESIUM HALIDE SOLUTION REDUCED IN BOTH MAGNESIUM AND BORON VALUES,REACTING SAID SUPERNATANT MAGNESIUM HALIDE SOLUTION WITH AN ADDITIONALQUANTITY OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF DOLIME ANDQUICKLIME IN AMOUNTS SUFFICIENT TO PRECIPITATE NO MORE THAN 95 TO 99% OFTHE REMAINING MAGNESIUM IN SAID SUPERNATENT MAGNESIUM HALIDE SOLUTIONPRECIPITATING A MAGNESIUM HYDROXIDE FRACTION HAVING A FINER PARTICLESIZE THAN THE INITIAL FRACTION OF MAGNESIUM HYDROXIDE AND SEPARATINGSAID FINER MAGNESIUM HYDROXIDE FRACTION FROM SAID MOTHER LIQUOR WHICH ISSUBSTANTIALLY REDUCED IN MAGNESIUM VALUES.